A solar cell is designed to convert light energy into electric power by means of a photovoltaic effect, and typically has a structure in which a p-type semiconductor and an n-type semiconductor are joined together, and an electrode and others are attached thereto.
An operating characteristic of a solar cell changes in response to changes in insolation condition or temperature, and a relationship between a voltage applied to a solar cell and a current flowing through the solar cell and a relationship between the applied voltage and an electric power generated from the solar cell are generally represented by operating characteristic curves depicted in FIG. 1.
As shown by the operating characteristics in FIG. 1, a solar cell is capable of generating a maximum electric power at a certain voltage. Thus, from a viewpoint of maximally utilizing solar energy in a solar power generation system using a solar cell, a solar cell module needs to be adjusted to operate at a maximum power point (MPP) by using a power conditioner or the like.
In the case where a plurality of solar cell modules are used in the form of a string constructed by connecting them in series, the solar cell modules can be partially shaded (shadowed) (this shading (shadowing) will hereinafter be referred to as “partial shading”), and this causes a variation in characteristics of the solar cell modules, thereby leading to a risk that the shaded solar cell module (hereinafter referred to as “shaded module”) is inversely biased.
Generally, with a view to preventing the inverse bias, the string is often used in a state in which a bypass diode is connected in parallel to each of the solar cell modules forming the string. In this case, however, it is known that, during occurrence of partial shading, the bypass diode connected to a shaded module is brought into conduction, and thereby the shaded module becomes unable to generate electric power, resulting in significant reduction of utilizable electric power.
Moreover, depending on an occurrence state of partial shading, a plurality of MPPs (points B and C in FIG. 2) appear on an operating characteristic curve of the whole string, and thereby a power conditioner is likely to adjust the string to operate at a non-optimal point (point C in FIG. 2) different from the true MPP (point B in FIG. 2).